Ch. 9 Linear Momentum.

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3Momentum (p)Momentum is a measure of how hard it is to stop or turn a moving object.

4What characteristics of an object would make it hard to stop or turn?

5Calculating Momentum For one particle p = mvNote that momentum is a vector with the same direction as the velocity!For a system of multiple particlesP = Spi add up the vectorsThe unit of momentum is…kg-m/s or Ns

6Sample Problem: Calculate the momentum of a 65-kg sprinter running east at 10 m/s.

7Sample Problem: Calculate the momentum of a system composed of a 65-kg sprinter running east at 10 m/s and a 75-kg sprinter running north at 9.5 m/s

8Change in momentum (DP)Like any change, change in momentum is calculated by looking at final and initial momentums.Dp = pf – piDp: change in momentumpf: final momentumpi: initial momentum

11Impulse (J)Impulse is the product of an external force and time, which results in a change in momentum of a particle or system.J = F t and J = DPTherefore Ft = DPFt = pf – piUnits: N-s or kg m/s (same as momentum

14Sample Problem: Suppose a 1Sample Problem: Suppose a 1.5-kg brick is dropped on a glass table top from a height of 20 cm. a) What is the magnitude and direction of the impulse necessary to stop the brick?

15b) If the table top doesn’t shatter, and stops the brick in 0b) If the table top doesn’t shatter, and stops the brick in 0.01 s, what is the average force it exerts on the brick?

16c) What is the average force that the brick exerts on the table top during this period?

17This force acts on a 1. 2 kg object moving at 120. 0 m/sThis force acts on a 1.2 kg object moving at m/s. The direction of the force is aligned with the velocity. What is the new velocity of the object?

18Law of Conservation of MomentumIf the resultant external force on a system is zero, then the vector sum of the momentums of the objects will remain constant.SPbefore = SPafter

19Sample problem: A 75-kg man sits in the back of a 120-kg canoe that is at rest in a still pond. If the man begins to move forward in the canoe at 0.50 m/s relative to the shore, what happens to the canoe?

20External versus internal forcesExternal forces: forces coming from outside the system of particles whose momentum is being considered.External forces change the momentum of the system.

21Internal forces cannot change momentum of the system.Internal forces: forces arising from interaction of particles within a system.Internal forces cannot change momentum of the system.

22An external force in golfThe club head exerts an external impulsive force on the ball and changes its momentum.The acceleration of the ball is greater because its mass is smaller.

24ExplosionsWhen an object separates suddenly, as in an explosion, all forces are internal.Momentum is therefore conserved in an explosion.There is also an increase in kinetic energy in an explosion. This comes from a potential energy decrease due to chemical combustion.

25Recoil Guns and cannons “recoil” when fired.This means the gun or cannon must move backward as it propels the projectile forward.The recoil is the result of action-reaction force pairs, and is entirely due to internal forces. As the gases from the gunpowder explosion expand, they push the projectile forwards and the gun or cannon backwards.

26Sample problem: Suppose a 5Sample problem: Suppose a 5.0-kg projectile launcher shoots a 209 gram projectile at 350 m/s. What is the recoil velocity of the projectile launcher?

27Sample Problem: An exploding object breaks into three fragments. A 2Sample Problem: An exploding object breaks into three fragments. A 2.0 kg fragment travels north at 200 m/s. A 4.0 kg fragment travels east at 100 m/s. The third fragment has mass 3.0 kg. What is the magnitude and direction of its velocity?

28CollisionsWhen two moving objects make contact with each other, they undergo a collision.Conservation of momentum is used to analyze all collisions.Newton’s Third Law is also useful. It tells us that the force exerted by body A on body B in a collision is equal and opposite to the force exerted on body B by body A.

29CollisionsDuring a collision, external forces are ignored. The time frame of the collision is very short. The forces are impulsive forces (high force, short duration).

32Perfectly Inelastic (stick together)Objects stick together and become one object.Deformation occurs, kinetic energy is lost.

33(Perfectly) Inelastic CollisionsSimplest type of collisions.After the collision, there is only one velocity, since there is only one object.Kinetic energy is lost.Explosions are the reverse of perfectly inelastic collisions in which kinetic energy is gained!

34Sample Problem: An 80-kg roller skating grandma collides inelastically with a 40-kg kid. What is their velocity after the collision?How much kinetic energy is lost?

35Sample Problem A fish moving at 2 m/s swallows a stationary fish which is 1/3 its mass. What is the velocity of the big fish after dinner?

36Sample problem: A car with a mass of 950 kg and a speed of 16 m/s to the east approaches an intersection. A 1300-kg minivan traveling north at 21 m/s approaches the same intersection. The vehicles collide and stick together. What is the resulting velocity of the vehicles after the collision?

37Conservation of MomentumSample Problem: Suppose three equally strong, equally massive astronauts decide to play a game as follows: The first astronaut throws the second astronaut towards the third astronaut and the game begins. Describe the motion of the astronauts as the game proceeds. Assume each toss results from the same-sized "push." How long will the game last?